Analyzing the Influence of Substituents on Proton Tautomerization-A Comparison of Tetra-tert-butyl Phthalocyanine Isomers

We present a comparative study of tetra-tert-butyl phthalocyanine (ttbPc) isomers on a Ag(111) surface by means of low-temperature scanning tunneling microscopy to analyze the influence of substituents on tautomerization, a proton transfer reaction, in single molecules. By comparing ttbPc with the well-studied phthalocyanine (H2Pc) molecule, we demonstrate decoupling from the surface by the tert-butyl substituents. A comparison between the four ttbPc isomers, which naturally exist because of different bonding positions of the tert-butyl groups on the macrocycle, reveals a significant influence of the structural differences on their tautomerization behavior, as evidenced by a switching rate which varies by up to a factor of four between ttbPc isomers. These findings can be understood by an energetic landscape of the proton switch which links the binding distance of tert-butyl groups with the height of potential barriers. This model is supported by the analysis of two types of deprotonated ttbPc molecules with the molecular nanoprobe technique.